He was the current Garrett's previous incarnation.
The man who would become known as the Sneak Thief was originally taught and trained by a secretive order known as the Keepers. Preferring to apply his unusual skills for personal gain, the thief turned to a life of crime rather than being inducted into the order.
It was during the early part of his career that the thief became caught up in the schemes of the Old God known as the Trickster. Losing his eye to the Trickster's trap, the thief barely survived this encounter, thanks to the assistance of the Keepers. With the help of the Hammerite order, the thief proceeded to infiltrate the Trickster's lair and put an end to the Old God once and for all.  This sequence of events would become known as the First Dark Age.
With the Trickster's defeat came a period known as The Metal Age, during which an extremist wing of the Hammerite order, known as the Mechanists, came to power. Their leader, Father Karras, had constructed a mechanical eye replace the one the thief had lost, in the hopes that the Sneak Thief would aid the Mechanists in their mission to purge the land of all organic life. This was not to be, and the thief soon brought an end to Karras' plot, as well. The Metal Age would also be recorded as the Second Dark Age.
With both the Pagans who had worshipped the Trickster and the majority of the Hammerites who had joined the Mechanists having suffered great blows to their numbers, the balance of power in the City had finally shifted to favor the more secretive Keepers. In time, it became evident that their long charge of secretly controlling the City's power structure using Glyphs had caused the Keepers themselves to become corrupted by a bureacratic lust for power. This was no more obvious than when a rogue Keeper by the name of Gamall revealed herself to be the prophesied "Brethren and Betrayer." Having kept herself alive for hundreds of years though Glyph magic, she proceeded to slaughter most of the Keepers, severely reducing their number. Revealed to be the One True Keeper, the Sneak Thief managed to activate the Final Glyph and break Gamall's power. This victory came at a heavy price, for the Final Glyph's magically erased all other Glyphs from existence, thereby returning the primal energy to the earth and cutting off access to its use.
This final sequence of events was initially known as the Third Dark Age, though it later become known simply as the Dark Age, referred to as a "time of woe" for which the Sneak Thief would be forever blamed. Without access to the Glyphs, the Keepers had lost the ability to influence the City's power structure. By the time of the City's next incarnation, House Northcrest had risen to prominence, its sons serving as Baron for several generations.
In Bry459, Edwina Moira, who had encountered the Sneak Thief during the events of the Third Dark Age, converted her manor home into an asylum to study and treat individuals with criminal tendencies, such as the Sneak Thief. During the reign of House Northcrest, however, Baron Ulysses saw in the asylum the potential to remove the degenerate from the City's streets, and the hospital eventually became more likened to a prison. It was around this point in time that the Sneak Thief was finally captured, and incarcerated in the asylum's underground prison. During his stay, the thief made multiple escape attempts, hiding various treasures throughout the asylum, which included coins from the old City and miraculously, a Montonessi painting,  despite the artist not being born for hundreds of years later. This latter treasure may have been obtained through some lingering Glyph magic that the thief had access to.
It is ultimately unknown whether the Sneak Thief ever managed to escape the asylum, but during one of these escape attempts, he lost his infamous mechanical eye. The device would remain in the old prison for many centuries, before finally being retrieved by the Sneak Thief's reincarnation.
- Obtaining all of the Sneak Thief's caches awards the achievement "Old Habits Die Hard."
- The Spectral Aspect statue, which "looks familiar," may also be a reference to the original Sneak Thief.
- It's possible that the Sneak Thief may still haunt Moira Asylum, which would explain the Montonessi painting suddenly appearing on its own, along with the ghostly echo, presumably that of an orderly, yelling, "The thief is loose!"